The present invention relates to rechargeable electrochemical energy storage systems, particularly such systems comprising complementary electrodes capable of reversibly intercalating, alloying, or otherwise alternately combining with and releasing lithium ions in electrical energy charge and discharge operations. The invention comprises, in its preferred embodiments, high capacity lithium battery cells comprising metal nitride electrodes which provide exceptionally high, stable discharge capacity in such cells.
Early rechargeable lithium battery cells relied primarily on metallic lithium electrodes, but disadvantages associated with recharging of such cells, particularly the formation of dendrites which led to shorting within the cell, resulted, in addition to resident dangers, in limited useful cycle life of these cells. Lithium alloys with metals such as tin or aluminum showed some promise of improvement from the dangerous conditions attributed to pure lithium metal; however, the relatively large expansion fluctuations exhibited by these materials during cycling resulted in intraparticle damage which ultimately defeated initial cell capacity gains.
Carbonaceous electrode materials, such as petroleum coke, hard carbon, and graphite, have been widely investigated and are regularly employed in lithium and lithium-ion cells, but these materials are limited in volumetric capacity and present other difficulties, such as their contributing to the instability and degradation of electrolyte compositions. Investigators have turned in part to employing lithiating electrodes comprising oxides of Sn, Si, Sb, Mg, and the like and have had some success in avoiding the drawbacks seen in carbon materials, but cycle life of these cells has lacked significant note.
Some specialized lithiation materials, such as oxides of non-alloying transition metals and the amorphized lithiated nitrides of transition metals, have also been investigated with varying success in capacity stability and cell voltage output. For example, the latter materials, described by Shodai et al. in U.S. Pat. No. 5,834,139, have reportedly exhibited good capacity and cycling stability; however, cell output voltage is significantly higher than desired in commercial implementations, and, of greater import, these active electrode materials are reactive in ambient atmosphere and require careful attention to controlled environment to enable practical use.
In the present invention it has been discovered that an active electrode material comprising a crystalline nitride of a non-alkali or non-alkaline earth metal, which may be lithium-alloying, such as zinc, or non-alloying, such as copper, and may include others, such as silicon, aluminum, and the like, provides, in combination with an active electrode material source of lithium ions and an intervening electrically insulative, ion-conductive separator typically incorporating a non-aqueous electrolyte composition comprising a dissociable lithium salt, a rechargeable battery cell exhibiting superlative cycling stability and exceptionally high capacity. Particularly when compared with widely used carbonaceous electrode materials, the metal nitride cell electrodes of the present invention yield gravimetric capacities which range from about 70% to 140% that of graphite, while the volumetric capacities resulting from these new electrode materials extraordinarily range from about 285% to 530% of the 740 mAh/cm3 of graphite.